Adjust linting and improve code quality

[infinityofspace]

Improve the overall code quality by properly using linting:

• simplified linting workflow by using only a single job matrix
• separated dev and prod dependencies to reduce the docker image size and general pip install size
• enabled more linting flake8 rules instead of only a very small subset
• improved the TXTUpdateError to contain all required information to handle the exception
• moved some internal object fields to private state